Cathode



July 1o, 1934.

W. T. MlLLlS CATHODE Filed March 24,' 1932 7% link IIII LIlfr /00 Volts.

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WITNESSES:

` TT'ORNY Patented July 10, 1934 UNTED srA'rss maaar ortica CATHODE Application March 24, 1932, Serial No. 600,866

1 Claim.

My invention relates to cathodes and it has particular relation to oxide coated cathodes of thermionic devices.

Anobject of my invention is to increase the 5 thermionically active content of an oxide coated lament.

Another object of my invention is to provide additional free alkaline earth metal in the coating of an oxide coated lament.

More specically stated, an object of my invention is to increase the thermionically active content of an oxide coated llament by adding barium azide to the coating of the lament in the process of manufacture and to reduce this barium azide to free metallic barium in the coating on the cathode of the completed tube.

Other objects of my invention will become evident from the following detailed description taken y in conjunction with the accompanying drawing, 20 in which Figure 1 is a View in elevation of a tube embodying my invention with part of the bulb broken away for the sake of clearness,

Fig. 2 is a view in elevation of a lament and its coating with part of the coating removed to show the core, and

Fig. 3 is a series of curves disclosing the superiority of the coating of my invention over the standard coatings.

The electron emission from oxide coated cathodes is due to the presence of free alkaline earth metal in such cathodesv. However, the amount of this free alkaline earth metal, generally barium, is usually very small in the stand- 35 ard coating now in use since the decomposition rate is very low.

This standard coating is usually applied in the form of carbonates of barium and strontium or of these two in combination with carbonates of 140 calcium. The carbonates are then reduced by heat to the oxide and a small amount of free barium is present upon the reduction of the carbonates. Sometimes, however, an activation process is necessary to form the free barium.

If the coating were entirely of free barium, then the cathode would become an emitter but owing to the high evaporation rate of the barium, the emission would soon disappear. Accordingly, the oxide is necessary to retard the evapora- 50 tion of the barium.

An object of my invention is to supply a morev abundant amount of free barium or other alkaline earth metal in an oxide coating so that the amount of emission, in respect to temperature, is

1.55 increased without approaching such a rate of (Cl. Z-27.5)

decomposition that this more abundant free alkaline earth metal is exhausted.

Broadly stated my invention deals with adding a readily decomposable compound of an alkaline earth metal which decomposes by heat to form a free alkaline earth metal plus an inactive residue, preferably in the form of a gas or vapor, to facilitate its removal from the device.

My invention specifically deals with adding barium azide (BaNe) to the usual or standard barium oxide and strontium oxide coated cathodes. This barium azide is a compound of barium which is readily decomposed. The decomposition is effected in air at a temperature of 225 C. and is probably less in a high vacuum. The decomposition is of the iorm: BaN+HeatzBa+3Na The barium azide may be mixed with the carbonates of barium and/or strontium, and/or calcium if desired, and with the use of a nitro-cellulose binder, preferably nitro-cellulose with a solvent such as amyl acetate, applied to the conductor, such as a lament or sleeve, in the form of a spray. Other methods such as dipping may be used but the spray method is preferred on account of the low temperature of decomposition of the barium azide.

A representative coating may be as follows:

25 gms. of BaCOs 25 gms. SrCGs 10 gms. BaNs 75 C.C.+nitrocellulose binder.

The percentage of the barium azide from the coating is not critical but may be varied from .5% up to 85% of the mixture. Ii desired, calcium carbonate may be added to the mixture or may replace the strontium carbonate. The barium azide may also replace the barium carbonate in the coating.

To ensure a good mixture and to obtain a fine enough emulsion for spraying, it is preferable to grind the carbonates together. Of course, the rate of grinding should not be too rapid to cause the barium azide to decompose by friction.

The coating is then dried on the lament or other shaped conductor at room temperature or in a low temperature oven at about 100 C. Fig. 2 discloses a coating 10 on a lament 11 and thus forming a cathode 12.

The coated cathode 12 is then incorporated in a tube such as the tube 13 of Fig. 1 having the grid 14 and the plate 15. The tube, including the coated cathode, then undergoes the usual and well known high vacuum treatment. This high vacuum treatment decomposes the carbonates into oxides, and also decomposes this barium azide to free metallic barium in the barium and strontiiun oxide.

, This results in the filament in Fig. 1 having a coating 1l o1" the oxides of barium and strontium permeated abundantly with free metallic barium. In making reference to the oxides o barium and strontium, it is, of course, understood that the oxide of calcium may also be included in the coating, if desired, preferably by mixing calcium carbonate with the carbonates oi barium and strontium and combining this with the barium azide, to produce my oxide coating as explained above.

The cathode having this increased electron emitting content of free barium has decided advantages over the mere oxide coating and some of these advantages are graphically illustrated by the curves in Fig. 3. It is possible that the en hanced activity is due to the presence of at least twice as much free barium as in the present standard oxide coating.

My coating will be reducible at the normal operating temperature of an oxide coating such as 4 watts per square centimeter. The barium azide has a much lower decomposition temperature than the carbonates in the activation of the coating. Accordingly, the presence of a large quantity of barium in the coating permits its normal emission at greatly reduced temperatures or enhanced emission at the usual operating temperature. This means lower operating wattages are required. This operation at lower temperature also means, of course, greater efficiency and longer life for a cathode. It is well known that the operation of lament cathodes at high temperatures is the major cause of short lives of tubes. EX- perimentally, I have found that a set of UX281 tubes having my coating on the laments were capable of giving 190 milliamperes with 3.5 volts on the lament and 100 volts on the plate, and, at this lament voltage, the temperature was too low to give a visible indication that the filament was hot, whereas the standard coating normally gives 1'70 milliamperes at 7.5 volts on the lanient and 100 volts applied to the plate.

The presence of a large amount of free barium metal in the coating increases its conductivity and results in a decrease in the impedance of the tube.

After thorough tests, I have also found no evidence at all of reverse emission. The oxide part of the coating holds the barium on the lament and prevents it from vaporizing onto the other parts of the tube, such as the grid and anode, which is the cause of the above-mentioned reverse emission.

Although I have described particularly the application of my new coating to a ilament, it is obvious that my coating can be applied to other forms of conductors or supports such as the Well known ribbon conductors or sleeves. This core may be of nickel, platinum, platinum iridium, platinum rhodium, or platinum nickel, but I pre- Yfer to use an alloy of nickel with cobalt and ferrotitanium such as described in the copending application o Erwin F. Lowry, ASerial No. l44,91l, lod October .28, 1926.

Although I have shown and described certain specic embodiments of my invention, .I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claim.

I claim as my invention:

The method of increasing the electron emissive content of a cathode coating of the compounds of barium and strontium which comprises mixing barium azide with said barium and strontium compounds, applying said mixture to a conductor, drying said mixture on said conductor at a low temperature, incorporating said conductor in a vacuum tight container and treating said coating to reduce said barium azide to free metallic barium in said coating.

WALTER T. MILLIS. 

